Tecmo Rygar arcade pcb repair

Game played but had no sound.  The audio amp (M51516L) was getting 12V ok, but produced absolutely no hum or hiss which is a good sign the amp itself has failed.  Before replacing I tried the usual trick of attaching power speakers to the preamp outputs but this didn’t give any sound either.  The audio CPU, ROM and RAM all tested good so I moved onto the YM3526 sound chip.  The IRQ pin was pulsing which is a good sign it’s working as the CPU must program the IRQ timer for this to happen.  However, a scope on the digital audio output stream showed it wasn’t trying to play any sound.

My next theory was the main CPU wasn’t telling the audio CPU to play anything – most games of this era use an 8 bit latch to pass sound commands.  The main CPU writes the audio command into the latch then signals an interrupt on the audio CPU to signal command data is ready.  A logic probe on the audio CPU NMI line showed it pulsing whenever a sound should play (such as coin insertion) so I took some guesses at what chip the latch was and found a nearby Fujitsu LS374 where the clock also pulsed on coin insertion (main CPU writing the command) as well as output enable (audio CPU reading the command).   Sound worked with a known good LS374 piggybacked over the top, so the Fujitsu chip had failed and was likely passing 0xff (all 8 bits high) for all audio commands.

After the LS374 and M51516L were replaced everything worked again.